[1.4.0] hidden recipient vs. ID 00000000

[Florian Weimer]

* David Shaw:

>> All-0 is not a valid V3 key ID because its LSB is not set.  All-1 is
>> theoretically valid, but rather unlikely (it imposes rather strict
>> requirements on the lower bits in both prime factors).
>
> True, but it doesn't matter in this case since all-0 and all-1 are
> both valid in the context of the key ID in a session key packet since
> v4 keys can be all-0 or all-1.

But this is extremely unlikely.  Currently, it's not computationally
feasible to create such V4 keys.  IMHO, RFC 2440bis should even outlaw
generation of keys which such key IDs.